Study on dominant heat transfer mechanism in vertical smooth/finned-tube thermal energy storage during charging process

Thermal energy storage (TES) is an indispensable part of solar energy utilization system. This paper intends to explore the role of heat conduction, natural convection and secondary flow on the charging performance of phase change material in vertical smooth/finned-tube TES respectively. It was found that heat conduction is dominant in smooth-tube TES. The role of natural convection is to cause non-uniformity of melting rate of the lower and upper part. Meanwhile, secondary flow is dominant in finned-tube TES due to the generation of vortices between adjacent fins. Fin material has little effect on the melting characteristic of phase change material when fin pitch is larger than 40 mm in this study. Therefore, nonmetallic fin and cheap metallic fin could be used in finned-tube TES to increase thermal storage density and reduce cost of investment. Compared to conduction model, the total melting time was reduced by 9.7% and 34.2% in smooth-tube TES and aluminum finned-tube TES when convection model is considered in the computation. Besides, the heat conduction intensity and secondary flow intensity were explored in aluminum finned-tube TES. The heat conduction enhancement rate φ was defined to characterize the heat conduction intensity. With the fin pitch decreasing from 40 mm to 8 mm, the heat conduction enhancement rate increases from 15.2% to 39.6%, indicating that heat conduction plays a crucial role in the finned-tube TES. The aluminum finned TES exhibit the best thermal storage characteristic as a result of the best heat transfer enhancement and smaller density.